Linear, branched and cross-linked

All the polymers discussed so far have linear chain structures. But not all polymers are linear.

Linear

Long chains lying side by side (figure 14.5).

Branched

Polymer molecules joined to the sides of the main chain of a linear polymer (figure 14.6).

Cross-linked

Linear chains tied to one another by cross chains (figure 14.7).

Figure to be added

Figures 14.5-14.7 — four wavy parallel chains (linear); the same chains with short side branches sticking out (branched); the same chains joined to one another by short vertical links (cross-linked).

Why structure changes behaviour

Linear chains are like uncooked spaghetti in a packet: they can lie close together, so the material can be quite dense and strong, and when warmed the chains slide past one another and the plastic softens and can be moulded again. Branches keep the chains further apart, so the material is lighter and softer — the difference between the soft polythene of a bag and the hard polythene of a crate. Cross-links tie the chains together, so they can no longer slide. A cross-linked polymer does not melt on heating and keeps its shape; this is why the summary notes that the shape of some polymers can be changed by heat while others cannot.

You can see the difference in everyday things. A polythene bag stretches and tears — its chains slide. A plastic bucket handle bends back into shape. A car tyre or a bakelite switch does not soften even when hot, because its chains are tied together. So when a question asks why one plastic can be melted and remoulded and another cannot, the answer is in the chains: free to slide, or tied.

Branches and cross-links differ

A branch hangs off one chain; a cross-link joins two chains together. Only cross-links stop the chains sliding apart.

Linear, branched, cross-linked

By structure, polymers are linear, branched (side chains) or cross-linked (chains tied to each other).